The man skilled in the art knows that, before proceeding to an operation of shaping semi-finished metallurgical products, the basic metal produced has to be subjected to treatment to remove therefrom dissolved gases and non-metallic impurities that it contains, the presence of which would detrimentally affect the desired properties and the easy solidification of the manufactured articles.
Two main treatment processes are known at the present time: the first comprises passing the liquid metal through inert or active filtration agents which retain the impurities, either mechanically or as a consequence of chemical reactions or by combinations thereof, while the second method involves using inert or reactive gases or mixtures thereof which are mixed by stirring more or less intensively with the liquid metal, with or without substances, such as fluxes, being present. However, these two main methods may be used in combination with each other.
In the second method, many alternative forms have been advanced, relating interalia to the manner of introducing ducing the gas into the bath of liquid metal, and the way of achieving greater or lesser dispersion of the gases in the metal. Thus, in French patent No. 1,535,953, the gas is introduced into the bath by a plunger arrangement, the lower part of which is provided with a rotary device for producing the stirring effect and distributing the gas over a large area of the bath.
In French patent No. 2,063,906, the gas is blown into the molten metal by means of a lance having a water-cooled double jacket.
In French patent No. 2,156,014, gases are injected in the form of small discrete bubbles by means of a device comprising a rotary shaft which is fixed with respect to a bladed rotor, and a fixed sleeve which is disposed around said shaft and which is connected at the lower end to a bladed stator; the shaft and the sleeve are separated by an axial-passage in which the gases are carried and introduced at the level of the blades where they are subdivided into small bubbles and brought into intimate contact with the metal which is agitated by the rotor.
In French patent No. 2,200,364, the gas is introduced at the center of rotation of a turbine agitator and brought into contact with the liquid metal under conditions of agitation which are such as to avoid any emulsification.
Many solutions have been proposed. However, each of those suffers from disadvantages. Thus, the apparatuses which involve injection of gas by way of an agitator have recourse to the use of gas distribution ducts which are disposed within the agitator, which complicates design of the arrangement and results in delicate systems which are expensive to maintain; moreover, such ducts are the cause of substantial pressure drops which limit the speed of injection of the gas into the bath. In addition, the speed of rotation of such arrangements and consequently their capability for dispensing the gases and agitating the bath is limited by the appearance of imbalance effects and abrasion phenomena. Certainly, it is possible to take steps against such imbalance phenomena by supporting the shafts of the agitators at their lower ends, but in that case it is necessary either to provide support bearings which operate in contact with liquid metal at a relatively high temperature or to extend the shafts to the outside of the metal container and provide them with rotary sealing means. These are solutions which give rise to technological difficulties that are often impossible to overcome.